The present invention relates to heavy duty industrial gas turbines and, in particular, to a burner for a gas turbine including a fuel/air premixer and structure for stabilizing pre-mixed burning gas in a gas turbine engine combustor.
Gas turbine manufacturers are regularly involved in research and engineering programs to produce new gas turbines that will operate at high efficiency without producing undesirable air polluting emissions. The primary air polluting emissions usually produced by gas turbines burning conventional hydrocarbon fuels are oxides of nitrogen, carbon monoxide, and unburned hydrocarbons. It is well known in the art that oxidation of molecular nitrogen in air breathing engines is highly dependent upon the maximum hot gas temperature in the combustion system reaction zone. The rate of chemical reactions forming oxides of nitrogen (NOx) is an exponential function of temperature. If the temperature of the combustion chamber hot gas is controlled to a sufficiently low level, thermal NOx will not be produced.
One preferred method of controlling the temperature of the reaction zone of a combustor below the level at which thermal NOx is formed is to premix fuel and air to a lean mixture prior to combustion. The thermal mass of the excess air present in the reaction zone of a lean premixed combustor absorbs heat and reduces the temperature rise of the products of combustion to a level where thermal NOx is not formed.
There are several problems associated with dry low emissions combustors operating with lean premixing of fuel and air in which flammable mixtures of fuel and air exist within the premixing section of the combustor, which is external to the reaction zone of the combustor. There is a tendency for combustion to occur within the premixing section due to flashback, which occurs when flame propagates from the combustor reaction zone into the premixing section, or autoignition, which occurs when the dwell time and temperature for the fuel/air mixture in the premixing section are sufficient for combustion to be initiated without an igniter. The consequences of combustion in the premixing section are degradation of emissions performance and/or overheating and damage to the premixing section, which is typically not designed to withstand the heat of combustion. Therefore, a problem to be solved is to prevent flashback or autoignition resulting in combustion within the premixer.
In addition, the mixture of fuel and air exiting the premixer and entering the reaction zone of the combustor must be very uniform to achieve the desired emissions performance. If regions in the flow field exist where fuel/air mixture strength is significantly richer than average, the products of combustion in these regions will reach a higher temperature than average, and thermal NOx will be formed. This can result in failure to meet NOx emissions objectives depending upon the combination of temperature and residence time. If regions in the flow field exist where the fuel/air mixture strength is significantly leaner than average, then quenching may occur with failure to oxidize hydrocarbons and/or carbon monoxide to equilibrium levels. This can result in failure to meet carbon monoxide (CO) and/or unburned hydrocarbon (UHC) emissions objectives. Thus, another problem to be solved is to produce a fuel/air mixture strength distribution, exiting the premixer, which is sufficiently uniform to meet emissions performance objectives.
Still further, in order to meet the emissions performance objectives imposed upon the gas turbine in many applications, it is necessary to reduce the fuel/air mixture strength to a level that is close to the lean flammability limit for most hydrocarbon fuels. This results in a reduction in flame propagation speed as well as emissions. As a consequence, lean premixing combustors tend to be less stable than more conventional diffusion flame combustors, and high level combustion driven dynamic pressure fluctuation (dynamics) often results. Dynamics can have adverse consequences such as combustor and turbine hardware damage due to wear or fatigue, flashback or blow out. Thus, yet another problem to be solved is to control the combustion dynamics to an acceptably low level.
Lean, premixing fuel injectors for emissions abatement are in common use throughout the industry, having been reduced to practice in heavy duty industrial gas turbines for more than two decades. A representative example of such a device is described in U.S. Pat. No. 5,259,184, the disclosure of which is incorporated herein by this reference. Such devices have achieved great progress in the area of gas turbine exhaust emissions abatement. Reduction of oxides of nitrogen, NOx, emissions by an order of magnitude or more relative to the diffusion flame burners of the prior art have been achieved without the use of diluent injection such as steam or water.
As noted above, however, these gains in emissions performance have been made at the risk of incurring several problems. In particular, flashback and flame holding within the premixing section of the device result in degradation of emissions performance and/or hardware damage due to overheating. In addition, increased levels of combustion driven dynamic pressure activity results in a reduction in the useful life of combustion system parts and/or other parts of the gas turbine due to wear or high cycle fatigue failures. Still further, gas turbine operational complexity is increased and/or operating restrictions on the gas turbine are necessary in order to avoid conditions leading to high-level dynamic pressure activity, flashback, or blow out.
In addition to these problems, conventional lean premixed combustors have not achieved maximum emission reductions possible with perfectly uniform premixing of fuel and air.
Dual Annular Counter Rotating Swirler (DACRS) type fuel injector swirlers, representative examples of which are described in U.S. Pat. Nos. 5,165,241, 5,251,447, 5,351,477, 5,590,529, 5,638,682, 5,680,766, the disclosures of which are incorporated herein by this reference, are known to have very good mixing characteristics due to their high fluid shear and turbulence. Referring to the schematic representation in FIG. 1, a DACRS type burner 10 is composed of a converging center body 12 and a counter rotating vane pack 14 defining a radially inner passage 16 and a radially outer passage 18 with respect to the axis 20 of the center body, co-axial passages each having swirler vanes. The nozzle structure is supported by an outer diameter support stem 22 containing a fuel manifold 24 for feeding fuel to the vanes of the outer passage 18.
While DACRS type fuel injector swirlers are known to have very good mixing characteristics, these swirlers do not produce a strong recirculating flow at the centerline and hence frequently require additional injection of non-premixed fuel to fully stabilize the flame. This non-premixed fuel increases the NOx emissions above the level that could be attained were the fuel and air fully premixed.
Swozzle type burners, a representative example of which is described in U.S. Pat. No. 6,438,961, the disclosure of which is incorporated herein by this reference, employ a cylindrical center body which extends down the center line of the burner. The end of this center body provides a bluff body, forming in its wake a strong recirculation zone to which the flame anchors. This type of burner architecture is known to have good inherent flame stabilization.
Referring to FIG. 2, an example of a swozzle type burner is schematically depicted. Air enters the burner 42 at 40, from a high pressure plenum, which surrounds the assembly, except the discharge end 44 which enters the combustor reaction zone.
After passing through the inlet 40, the air enters the swirler or ‘swozzle’ assembly 50. The swozzle assembly includes a hub 52 (e.g., the center body) and a shroud 54 connected by a series of air foil shaped turning vanes 56 which impart swirl to the combustion air passing through the premixer. Each turning vane 56 includes gas fuel supply passage(s) 58 through the core of the air foil. These fuel passages distribute gas fuel to gas fuel injection holes (not shown) which penetrate the wall of the air foil. Gas fuel enters the swozzle assembly through inlet port(s) and annular passage(s) 60, which feed the turning vane passages 58. The gas fuel begins mixing with combustion air in the swozzle assembly 62, and fuel/air mixing is completed in the annular passage, which is formed by a center body extension 64 and a swozzle shroud extension 66. After exiting the annular passage, the fuel/air mixture enters the combustor reaction zone where combustion takes place.
The DACRS and swozzle type burners are both well-established burner technologies. That is not to say, however, that these burners cannot be improved upon. Indeed, as noted above, the DACRS type burners do not typically provide good premixed flame stabilization. Swozzle type burners, on the other hand, do not typically achieve fully uniform premixing of fuel and air.